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Modules
Let's look at a typical crystalline silicon PV module. It
consists of a transparent top surface, an encapsulant, a rear
layer, and a frame around the outer edge. In most modules, the
top surface is glass, the encapsulant is ethyl vinyl acetate
(EVA), and the rear layer is Tedlar.
Front Surface Materials
— The front surface of a PV module must have a high
transmission in the wavelengths that can used by the solar
cells in the PV module. For example, for silicon solar cells,
the top surface must have high transmission of light having
wavelengths in the 350 to 1200 nm range.
Also, the reflection from the front surface should be
minimal. An antireflection coating to the top surface can
greatly reduce the reflection of sunlight, and texturing of
the surface can cause light that strikes the surface to stay
within the cells. Unfortunately, these textured modules are
not "self-cleaning," and the advantage of reduced reflection
is usually outweighed by losses due to dust sticking to the
surface.
The top surface should also be impervious to water, be able
to resist damage from hail impact, be stable under long-term
exposure to ultraviolet radiation, and have low thermal
resistivity. If water as liquid or a vapor is able to get
inside a PV module, it will cause corrosion of metal contacts
and interconnects, which will greatly shorten the lifetime of
the PV module. Also, the front surface often provides rigidity
for the module. |
The above solar water
pumping installations are using BP Solar panels to power the
highly efficient Lorentz Submersible Pumping system.
Typical systems like these installed in the Plains of South
Dakota may pump from 1-43 gallons per minute depending on
solar power input. |
Of several choices for a top surface material, a common choice
is tempered, low-iron glass, which is low cost, strong, stable,
highly transparent, impervious to water and gases, and has good
self-cleaning properties.
Encapsulant — An
encapsulant helps to hold together the top surface, PV cells, and
rear surface of the PV module. The encapsulant must be stable at
high temperatures and high levels of ultraviolet radiation. It
must also be optically transparent and have a low thermal
resistance. Ethyl vinyl acetate—or EVA—is the most commonly used
encapsulant. Thin sheets of EVA are inserted between the solar
cells and the top and rear surfaces. Heating this "sandwich"
causes the EVA to polymerize, thus bonding the module into one
piece.
Rear Surface — The
material used as the rear surface of the PV module must have low
thermal resistance and must prevent the ingress of water and
gases. In many modules, the rear surface material is a thin
polymer sheet, typically Tedlar.
Frame — A final
structural component of the module is the frame, which is
typically made of aluminum.
Flat-plate collectors,
which typically contain a large number of solar cells mounted on
a rigid, flat surface, can make use of both direct sunlight and
the diffuse sunlight reflected from clouds, the ground, and
nearby objects.
We add one final point. Because the amount of power produced by
a single cell is relatively small, designers group solar cells
together to form electrical modules that supply a more useful
level of voltage, current, and power. Solar cells may be connected
in series to produce higher voltages. This is accomplished by
connecting the positive terminal of one cell to the negative
terminal of the next cell. Cells may also be connected in parallel
to produce more current. This is accomplished by connecting the
positive terminal of the first cell to the positive terminal of
the next cell, and the negative terminal of the first cell to the
negative terminal of the second cell. |
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